# Aerodynamically - can a rocket be over 30 meters in diameter?

The past and current designs of rocket launchers have diameters up to 8-10 meters (Saturn-5, N-1, SLS, BFR), up to 13-14 meters for max cross-section dimension for Space Shuttle and Energia-Buran (launcher+orbiter).

I am curious:

Theoretically, for aerodynamic reasons - can a rocket launcher have a diameter of 30 meters and more? Will there be any insurmountable difficulties with air flow or something else? A rocket is supposed to launch from sea level in Earth's atmosphere.

I know two projects of launchers with diameter over 20 meters:

Sea Dragon (diameter 23 meters)

UR-900 (diameter 28 meters)

Both were never close to implementation, and I don't know how mature these projects were in aerodynamics.

• The largest diameter booster proposal I'm aware of was the Chrysler SERV. It was 29 meters in diameter at the base. en.wikipedia.org/wiki/Chrysler_SERV It was a contender to be selected for the Space Shuttle design, and would have been fully reusable. It was...different...but not considered unworkable AFAIK. Aug 12 '18 at 17:23
• new SciShow Space video Meet the Sea Dragon: The Biggest Rocket Ever Designed
– uhoh
Jul 13 '19 at 2:35

All other things being equal, if you maintain the basic long-and-pointy proportions of a rocket design, the aerodynamics actually get better as the rocket gets bigger.

Air resistance force grows generally with the square of linear scale, as do surface drag effects. However, those forces are working against the overall mass of the vehicle, which tends to grow as the cube of linear scale. Thus the bigger the rocket, the less relative impact of aerodynamics on it.

The size of rockets built so far has been mainly a function of construction and transport logistics, and payload size demand, not one of aerodynamics.

• Like drag, room for engines and nozzles at the bottom only scales as the square, so mass growing as the cube could be a problem. Luckily however, the question doesn't ask about engines or thrust.
– uhoh
Aug 12 '18 at 23:16
• Well, crap, I'm going to have to revisit my "how big can rockets get" answers in light of that. I think it wouldn't be an insurmountable problem at 30 meter diameter -- you could flare the base of the first stage and use mostly non-gimbaling (thus space saving) nozzles with a few gimbaling at the outside for thrust vectoring, and/or add jettisonable booster rings a la Atlas. Aug 12 '18 at 23:21
• A rocket scientist's work is never done.
– uhoh
Aug 13 '18 at 0:26

Sure, there's no reason in physics why rockets won't scale up. You just need a really tall rocket to get to a diameter of 30 m (rockets need to be much taller than they are wide to get decent aerodynamics).